JERUSALEM (Reuters) - Israel's military said on Thursday its special forces raided an underground missile production site in Syria in September that it said was primed to produce hundreds of precision missiles for use against Israel by the Iranian-backed Hezbollah.

The complex near Masyaf, in Hama province close to the Mediterranean coast, was "the flagship of Iranian manufacturing efforts in our region," Israeli military spokesperson, Lieutenant Colonel Nadav Shoshani told a briefing with reporters.

"This facility was designed to manufacture hundreds of strategic missiles per year from start to finish, for Hezbollah to use in their aerial attacks on Israel," he said.

He said the plant, dug into the side of a mountain, had been under observation by Israeli intelligence since construction work began in 2017 and was on the point of being able to manufacture precision-guided long-range missiles, some of them with a range of up to 300 km (190 miles).

"This ability was becoming active, so we're talking about an immediate threat," he said.

Details of the Sept. 8 raid have been reported in the Israeli media in recent days but Shoshani said this was the first confirmation by the military, which usually does not comment on special forces operations of this type.

At the time, Syrian state media said at least 16 people were killed in Israeli airstrikes in the west of the country.

Shoshani said the hours-long nighttime raid was "one of the more complex operations the IDF has done in recent years". Accompanied by airstrikes, it involved dozens of aircraft and around 100 helicopter-borne troops, who located weapons and seized documents, he said.

"At the end of the raid, the troops dismantled the facility, including the machines and the manufacturing equipment themselves," he said, adding that dismantling the plant was "key to ensure the safety of Israel".

Israeli officials have accused the former Syrian government of President Bahar al-Assad of helping the Lebanese-based Hezbollah movement receive arms from Iran and say they are determined to stop the flow of weapons into Lebanon.

As Assad's government crumbled towards the end of last year, Israel launched a series of strikes against Syrian military infrastructure and weapons manufacturing sites to ensure they did not fall into the hands of its enemies.

(Reporting by James Mackenzie; Editing by Andrew Heavens)